1. Field of the Invention
The present invention relates generally to the field of Global Positioning System (GPS) receivers, and more specifically, to the field of GPS receivers that achieve significant reduction of the multipath error signals.
2. Description of the Background Art
In the available art related to GPS receivers, the multipath errors originate with contamination of GPS signals by delayed versions of these signals. For some applications using either pseudo-range or carrier phase observables, multipath is the dominant error source. The most direct approach for reducing this error is to select an antenna site distant from reflecting objects, and to design antenna/back plane combinations to further isolate the antenna from its surroundings. In some cases, however, antennas must be located in relatively poor sites, and other techniques for code multipath reduction are required.
In the U.S. Pat. No. 5,414,729, issued to Fenton, a receiver for pseudorandom noise (PRN) encoded signals is disclosed.
The Fenton receiver consists of a sampling circuit, multiple carrier and code synchronizing circuits, and multiple correlators, with each correlator having a selectable code delay spacing. The time delay spacing of the multiple correlators is distributed around an expected correlation peak to produce an estimate of the correlation function parameters which vary with respect to multipath distortion. This information may be used in turn to determine the offset estimates for locally generated PRN reference code and carrier phase tracking signalsxe2x80x9d, Col. 3, lines 21-36. In another embodiment of the Fenton device, xe2x80x9cthe majority of the channels in a receiver can be left to operate normally, with one or more of the channels being dedicated to continuously sequencing from channel to channel to determine the multipath parameters for a partial PRN code being trackedxe2x80x9d, Col. 3, lines 44-49. Thus, the Fenton device includes a plurality of tracking satellite channels used to estimate the multipath parameters.
However, the Fenton device ""729 includes a number of limitations. (1) The Fenton device does only tracking. After tracking, there is still a multipath blimp left in the correlation function that has to be removed by some other means. (2) The Fenton device uses the full correlation. Indeed, the Fenton device correlates through the entire chip period. (3) The Fenton device employs multiple correlators time delay spacing. This is equivalent to Early minus Late (Exe2x88x92L) response function that is limited to one chip time period. That is, the Fenton device is a xe2x80x9cnarrow correlatorxe2x80x9d device. If Fenton""s (Exe2x88x92L) response function is not limited to only one chip time period and is extended over two chip periods, the Fenton device would not be beneficial over the prior art at all. (4) Using the filter function approach, the xe2x80x9cnarrow correlatorxe2x80x9d property of the Fenton device can be described as follows: the amplitude of the Fenton""s filter function decreases when time increases between zero and two chip time periods.
Another such technique for code multipath reduction was disclosed by Rayman Pon in the U.S. Pat. No. 5,966,403, entitled xe2x80x9cCode Multipath Error Estimation Using Weighted Correlationsxe2x80x9d that was assigned to the assignee of the present patent application, and that was filed on Jul. 19, 1996. This patent is specifically referred to in the present patent application and is incorporated herein by reference as patent #3. In the above referenced patent #3 a standard tracking channel was disclosed. The code multipath reduction in the above referenced patent #3 was based on the utilization of the weighting and correlation means in the standard tracking channel that changed the magnitude and shape of the composite signal autocorrelation function to estimate and suppress the contribution of a multipath signal. (1) In the above referenced patent #3, only code tracking is performed. However, a multipath blimp left in the correlation function after code tracking can be removed by using an additional circuitry, as disclosed in the current patent application. (See discussion below). (2) The patent #3 employs a partial weighted correlation function that is partially non-zero during one chip time period. (3) The patent #3 employs the xe2x80x9cbroad correlatorxe2x80x9d approach including an (Exe2x88x92L) response function that is not limited to one chip time period and can be extended up to two chip time periods. (4) Using the filter function approach, the xe2x80x9cbroad correlatorxe2x80x9d property of the device of the patent #3 can be described as follows: the amplitude of the filter function of the patent #3 is flat and does not change when time increases between zero and two chip time periods.
What is needed is the technique that allows to generate and use the additional signals to modify certain intermediate signals in the standard tracking channel in order to minimize the contribution of multipath signals.
To address the shortcomings of the available art, the present invention provides an apparatus and a method that allows to generate and use the additional signals to modify certain intermediate signals in the tracking channel in order to minimize the contribution of multipath signals.
One aspect of the present invention is directed to an apparatus for use in decoding a composite signal including a satellite signal from a satellite and including a multipath distortion component. The apparatus comprises: (A) a tracking satellite channel circuit for tracking the composite signal from the satellite; and (B) an additional circuit for generating an additional signal. The additional signal is used to minimize the multipath distortion component.
In one embodiment, the tracking satellite channel circuit further comprises: (1) an inphase (I) input circuit for processing the input composite signal from the satellite and for generating an inphase (I) component of the input composite signal; and (2) a quadrature (Q) input circuit for processing the input composite signal from the satellite and for generating a quadrature (Q) component of the input composite signal. In the first embodiment, the additional circuit further includes: (3) an additional input (A) circuit for modifying the inphase (I) component of the input composite signal at input level; and (4) an additional input (B) circuit for modifying the quadrature (Q) component of the input composite signal at input level.
In the alternative embodiment, the tracking satellite channel circuit further comprises: (1) an inphase (I) local carrier reference circuit for generating an inphase component (I) of a local carrier reference signal; and (2) a quadrature (Q) local carrier reference circuit for generating a quadrature component (Q) of a local carrier reference signal. The additional circuit in the second embodiment further includes: (3) an additional local carrier reference (A) circuit for modifying the inphase (I) component of the local carrier reference signal; and (4) an additional local carrier reference (B) circuit for modifying the quadrature (Q) component of the local carrier reference signal.
In one additional embodiment, the tracking satellite channel circuit further comprises (1) a complex mixer circuit for mixing an inphase component (I) and a quadrature component (Q) of a locally generated carrier signal with an inphase (I) component and a quadrature component (Q) of the input sampled signal and for generating an inphase (1) component of a baseband sampled composite signal. In this embodiment, the additional circuit further includes: (2) an additional local carrier mixing (A) circuit for modifying the inphase (I) component of the baseband sampled composite signal and for generating an Early baseband sampled composite signal; and (3) an additional local carrier mixing (B) circuit for modifying the inphase (I) component of the baseband sampled composite signal and for generating a Late baseband sampled composite signal.
Yet, in more embodiment, the tracking satellite channel circuit further comprises: (1) an Early code reference circuit for locally generating an Early code reference signal; and (2) a Late code reference circuit for locally generating a Late code reference signal. In this embodiment, the additional circuit further includes: (3) an additional local code reference (A) circuit for modifying the Early code reference signal; and (4) an additional local code reference (B) circuit for modifying the Late code reference signal.
Yet, in another embodiment, the tracking satellite channel circuit further comprises: (1) an Early code correlation circuit for correlating the inphase (I) component of the baseband sampled composite signal with the locally generated Early code reference signal and for generating an Early component of a composite correlation signal; and (2) a Late code correlation circuit for correlating an inphase (I) component of the baseband sampled input composite signal with the locally generated Late code reference signal and for generating a Late component of a composite correlation signal. The additional circuit in this embodiment further includes: (3) an additional correlation (A) circuit for modifying the Early component of the composite correlation signal; and (4) an additional correlation (B) circuit for modifying the Late component of the composite correlation signal.
In the preferred embodiment, the additional circuit further includes: (1) a code NCO and Generator for generating a local code signal; (2) an Early flip-flop for generating an Early signal; (3) a Punctual flip-flop for generating a Punctual signal; (4) a Late flip-flop for generating a Late signal; (5) an Late-Late flip-flop for generating a Late-Late signal; (6) a first EX-OR gate for generating a (B) additional signal; and (7) a second EX-OR gate for generating an (A) additional signal. The apparatus further includes a computer for closing a code and a carrier tracking loops, and for minimizing the multipath distortion component by optimizing the additional signals.
Another aspect of the present invention is directed to a method for decoding a composite signal including a satellite signal and a multipath distortion component. The method comprises the steps of: (1) tracking the satellite signal by a tracking channel; (2) generating additional signals by an additional circuit; and (3) minimizing the multipath distortion component by optimizing the additional signals by a computer.